The presence of liquid in a tank which is exposed to dynamic conditions has been under review for quite some time. Such scenarios may occur in liquid propellant rockets, aircraft propellant tanks, ships, petroleum tankers, and other applications in which fluids are contained for transport. Sloshing of a liquid can cause the tank system to deviate due to the buildup of kinetic energy of the liquid and its consequent interaction with the walls of the container.
In aerospace technology, sloshing is a well-recognized problem, particularly in liquid propellant launch vehicles, which tend to have an enormous percentage of their initial weight as fuel. When slosh waves are allowed to freely oscillate, they have a tendency to reach resonance. At resonance, slosh waves generally have maximum amplitude. The forces of sloshing propellant can cause the spacecraft to nutate about its spin axis. Traditionally, thrust vector correction methods are used to correct the nutation in the spacecraft. However, the high magnitudes of propellant sloshing forces can overpower the corrections being made. This can result in an increase in nutation and complete loss of the control of the spacecraft.
Movement imparted to a liquid containing tank can cause the liquid to slosh within the tank. Liquid sloshing often results in the periodic motion of liquid with the free surface in a liquid container. The hydrodynamic forces exerted due to sloshing pose a risk to the structural integrity of the tank walls. This concern is especially augmented with the constantly increasing size of space vehicles and rocket vehicle propellant containers and the significant dynamic forces these containers are exposed to.
To counteract the effects of liquid slosh, various propellant management devices (PMD) have been proposed and placed into use that involve both passive structural devices and active mechanisms. Passive systems are understood to operative without outside influence. Active systems are understood to operate by influencing the liquid or elements of the tank system in a controllable manner. FIG. 1 illustrates known slosh reduction techniques including diaphragms, baffles, and acoustic damping. A diaphragm is one type of passive PMD that is implemented within the propellant tank itself. As the liquid propellant leaves the tank, the diaphragm compresses and lessens the amount of surface area in which the liquid would originally slosh.
A baffle is another type of passive PMD that creates a simple barrier that acts to restrict the physical motion of the liquid, thus damping the amount of slosh it can exhibit. The two main types of baffles are wall-fixed and floating. Each baffle type is accompanied by its own set of advantages and disadvantages. The primary advantage of floating baffles is the reduced weight over the wall-fixed counterparts. Additionally, wall-fixed baffles are limited in their placement with respect to the tank walls due to the presence of supporting structures which may be located on the tank walls. Floating baffles have shown some effectiveness in part because they interact by colliding with one another, thus absorbing the kinetic energy imparted to the fuel upon movement of the tank. Other baffle methods have some effectiveness by increasing the natural frequency of the tank sections and decreasing the wave amplitude at the free surface. Often, however, passive PMDs are effective across a limited range of propellant slosh frequency; they take up space in the fuel tank and are relatively heavy and bulky. Thus, the weight to operation range ratio of passive PMDs is poor.
One type of active damping mechanism physically creates waves that destructively interfere with the liquid propellant undergoing sloshing. By changing the frequency at which these waves are created, the magnitude of the sloshing waves can be controlled. However, unlike the passive PMDs, this active mechanism is not believed to have been applied to any commercial use.
Therefore there remains a need for additional mechanisms for damping of liquid slosh within a container.